U.S. Pat. No. 5,098,637 to Hendry discloses an internal gas pressure method that is used to produce hollow plastic molded parts. According to this prior art technique, a pressurized gas is injected into the plastic melt within the main molding tool cavity in order to form a hollow space in the interior of the molded parts. An overflow cavity is connected to the main cavity of the tool to receive any excess plastic displaced by the gas. The following process sequence is specified: plastic melt injection into the tool cavity; expelling a portion of the melt from the main cavity into the overflow cavity, by the introduction of the pressurized gas into the cavity; cooling the melt; venting the cavity; and the removal of the finished molded part from the molding tool.
European Pat. Document EP 0 321 117 B1 to Melea Limited proposes a similar method. Plastic melt is first placed into the cavity of the molding tool. Then pressurized gas is injected into the melt, which distributes the melt over the interior surfaces of the molding tool and forms the hollow space within the melt. Then follow the steps of cooling, venting, and removal from the mold. A portion of the melt is expelled from the main cavity into an overflow cavity while the melt or gas is being introduced into the main cavity, such that the overflow cavity first receives the gas, from which the gas then spreads into the main cavity.
A commonality between the two methods is that the main cavity and the overflow cavity are connected by a channel. This channel establishes a permanent fluidic connection between the two cavities.
The technology is developed further in German Pat. Document DE 39 13 109 C2 to Klochner Ferromatik. Here, the main cavity and the secondary cavity are connected through a channel which contains a valve, which makes it possible to disable or enable the connection between the main and secondary cavities. The corresponding method runs as follows. First, plastic melt is introduced into the mold cavity. The cavity is filled completely with the melt. After the plastic melt has begun to solidify at the walls of the cavity, the core of the plastic body, which is still liquid, is expelled by a pressurized gas into the overflow cavity.
A similar method is known from the European Pat. Document EP 0 438 279 A1 to Keter Plastic. The objective there is to perform the actual injection molding process under the high pressure that is typical for injection molding, and not under the comparatively low gas pressure that normally prevails with the interior gas pressure techniques. A seal is here provided at the end of the cavity, and this seal is initially closed. In this operating mode, the melt is injected into the cavity at high pressure, until the cavity is completely filled. After the melt begins to solidify, the seal is opened, and thus a connection to an overflow cavity is enabled. The liquid, melted core of the molded part is then expelled into this overflow cavity by pressurized gas.